The use of a torsional vibration damper conventionally has been an accepted way to absorb undesirable torsional vibrations from the vehicle engine or transmission input during operation of a motor vehicle. Initially, vibration dampers have been designed with springs in parallel and in series with a capacity to absorb torsional shock or vibration emanating from an engine or transmission input without the spring in the vibration damper being fully compressed to its solid height. However, as engines became more efficient and more powerful, the vibration dampers had to absorb increased torsional loads and the springs in the vibration dampers compressed to a greater extent to absorb the increased loads.
Because many of the springs were not designed to be fully compressed, many of these torsional vibration dampers would have a stop limit built in. However, the stop limit adds extra complexity to the torsional vibration damper and also takes room that can otherwise be used to house more shock absorbing springs.
Due to packaging constraints, the torsional vibration dampers increased capacity must be achieved without an increase in size. Hence, what is needed is a torsional vibration damper that has its springs designed to be durable and to be fully compressed thereby maximizing the capacity for vibration absorption and eliminating the necessity for a separate limit stop built into the torsional vibration damper.
A spring has been defined as a machine element for storing energy as a function of displacement. Force applied to a spring member causes it to deflect through a certain displacement thus absorbing energy. A spring may have any shape and may be formed from any elastic material. In particular, coil springs are commonly utilized in damping arrangements in clutch plates and torsional dampers.
The springs in the damper provide vibration control by reducing stiffness of a drive line. Additional spring wire stressed in torsion lowers the drive line stiffness. Thus, the springs, by lowering the stiffness of the drive line, provide vibration control from impulses or shocks in the driveline of an automotive vehicle between the vehicle engine and the transmission.
A helical spring is formed from a bar or wire, commonly with uniform cross section, which is wound in a helix. Usually, the last turn at each end of the spring is modified to have a planar end surface lying in a plane perpendicular to the helix axis, and force can then be applied to the bearing surfaces of the end coils to place the helix in compression. Springs may be classified into several varieties with the variation in the end coil providing the difference. A plain end for a coil spring has a constant pitch for both the active coils and the end coils with the wire cut off generally square to the axis of the wire. The end coils can then be ground square to the spring axis, providing flat bearing surfaces. On the other hand, a closed end spring has the final coil bent so that at least a portion of the end coil is in contact with the adjacent active coil of the spring. In this version also, the end coil can be ground flat to provide a bearing surface for the spring.
Commonly, springs that have been designed to be fully compressed are closed end springs that have their final coil having a pitch equal to the thickness of the wire such that non-torsional bending forces are minimized on each coil. In grinding this end coil, a compromise must be made with respect to the arcuate extent of the coil which is ground perpendicular to the axis of the spring. A full 360.degree. grind has been impractical. If the full 360.degree. of the end coil were to be ground, the end tip would be extremely thin and would be damaged or broken off by the forces exerted during the use of the spring. As a lesser arcuate sector of the end coil is ground, a greater tip thickness is produced, however, the seating area of the spring is also reduced and the increased thickness of the tip produces heavy loading on the first active coil when the spring is fully compressed. These conflicting limitations result in a very narrow range of acceptability of grinding and are difficult to meet in production.